The present invention relates to an imaging optical element that uses a single gradient index rod lens or a lens array in which a plurality of gradient index rod lenses are arranged in an array. More particularly, the invention is related to an optical imaging element that is adapted to eliminate flare beam by means of a field limitation stop member and that can eliminate the need for a flare-cut treatment by chemical etching on the external peripheral surface of the rod lens.
The gradient index rod lens is an optical element formed with parabolic refractive index distribution from the center toward the periphery thereof by carrying out, for example, an ion exchange treatment on a glass rod. Since optical beam propagates within the gradient index rod lens while meandering at a specific frequency, the gradient index rod lens has the function equivalent to an ordinary lens.
A rod lens of this kind may receive optical beam rays that enter into the rod lens at an angle larger than a numerical aperture of the rod lens. Since a glass rod, which is processed by means of an ion exchange treatment, has, at least in appearance, a smooth external surface as a result of the processing of fusing, spinning and then solidifying a glass, the beam rays entering at an angle larger than the numerical aperture are reflected by the external peripheral surface of the rod lens to propagate through the inside of the lens. When these beam rays are emitted from the lens, they become, what is called, flare beam. Flare beam causes a decrease in the resolution of a produced image. That is, the beam rays entering into the rod lens at an angle larger than the numerical aperture of the rod lens causes a noise on an image. Accordingly, countermeasures to prevent flare beam have been taken by carrying out a flare-cut treatment so as to form intentionally a rough surface on the external peripheral surface of the lens in order to prevent the beam rays which enter at a large angle from being reflected. In such a case, the flare-cut treatment to make a rough surface is carried out by chemical etching.
When refractive index distribution is formed within a glass rod by an ion exchange method, the refractive index distribution has a tendency to deviate from a configuration necessary for lens in the vicinity of the external periphery thereof. The above-described chemical etching is used for the purpose also of removing part of the external periphery wherein the configuration of refractive index distribution is deviated. The external peripheral surface of the glass rod after being processed by an ion exchange treatment is removed not only at a surface layer, but up to several microns in depth.
An example of an application of the gradient index rod lens is a rod lens array. The rod lens array is a beam convergent optical device that is so structured that a plurality of gradient index rod lenses are arranged parallel with each other in an array between two side plates disposed parallel with each other a distance apart and are combined into one unit by filling the clearances with black silicon resin by impregnation. The rod lens array is arranged so that a real erect image of real size is produced as a whole as a result of overlap of images produced by the neighboring lens elements. Since the rod lens array has a short optical length and does not require any inversion mirror, it has an advantage that it is possible to structure the device small in size. Consequently, the rod lens array is used in many optical scanning systems such as facsimiles and printers.
In the rod lens array described above, in order to prevent a decrease in resolution due to flare beam, gradient index rod lenses that are processes with a flare cut treatment by chemical etching on the external peripheral surface thereof are used.
As described above, in the related arts, in order to eliminate flare beam as well as to remove a portion poor in refractive index distribution, a chemical etching is carried out on the external periphery of the rod lens. However, as a consequence of the chemical etching, it is extremely difficult to reduce deviation in lens diameter. Particularly, the smaller in diameter of the rod lens, the more remarkable deviation appears in lens diameter.
In a rod lens array, due to such deviation in lens diameter, a fluctuation is caused in the lens array. As a result, unevenness in the produced image occurs.
An object of the invention is to provide an imaging optical element adapted to prevent a decrease in resolution due to flare beam without carrying out a chemical etching treatment on the external peripheral surface of the gradient index rod lens. Another object of the invention is to provide an imaging optical element in the form of a rod lens array that has little fluctuation in the array using the gradient index rod lenses uniform in lens diameter.
The invention is an imaging optical element including a gradient index rod lens and a field limitation stop member mounted on the lens surface at the incidence side thereof.
In the imaging optical element, it is preferable that the stop member is adapted so that the length t thereof is:
t greater than 1oxc2x7D/2X 
where, 1o: a lens working distance, D: a diameter of the rod lens, X: a radius of field of view, and whereby the incident angle xcex8 of beam relative to the center of the lens satisfies:
xcex8 less than tanxe2x88x921(X/1o). 
The image optical element is preferably constructed to have the gradient index rod lens of which external peripheral surface remains smooth surface without being processed by means of a flare-cut treatment.
Also, the invention is an imaging optical element that includes a rod lens array having such a structure that a plurality of gradient index rod lenses are arranged in an array and disposed between two side plates and combined into one unit by filling the clearances with a resin, and a field limitation stop member mounted on the lens surface at the incidence side thereof.
In the imaging optical element, the stop member is adapted so that the length t thereof is:
t greater than 1oxc2x7D/2X 
where, 1o: a lens working distance, D: a diameter of the rod lens, X: a radius of field of view, and whereby the incident angle xcex8 of beam relative to the center of the lens satisfies:
xcex8 less than tanxe2x88x921(X/1o) 
The imaging optical element is preferably constructed to have the gradient index rod lenses of which external peripheral surfaces remain smooth without being processed by means of a flare-cut treatment.
More preferably, the stop member is adapted so that the length t thereof is:
t greater than 1oxc2x7D/2Xxe2x80x2
where, Xxe2x80x2: a maximum height of object that a good image is obtained, and whereby the incidence angle xcex8 of beam relative to the center of the lens,
xcex8 less than tanxe2x88x921(Xxe2x80x2/1) 
is satisfied.
The present disclosure relates to the subject matter contained in Japanese patent application No. Hei. 2000-109022 (filed on Apr. 11, 2000), which is expressly incorporated herein by reference in its entirety.